The present invention relates to a method for yield tightening of screws to tighten a screw up to the maximum of elastic stress of the screw.
In the field of screw tightening, attention is now being focused on what is called a yield tightening method which tightens a screw up to the maximum of elastic stress of the screw itself, and there is also a growing tendency toward wide application of the method in the actual production line.
A physical phenomenon commonly called yield refers to a phenomenon that as external tensile force is applied to, for example, a rodlike object of metal, the external force and the elongation of the object are proportional to each other in the elastic area but in the plastic area only the elongation of the object increases although the external force does not substantially increase. In graphical terms, letting the external tensile force be represented on the ordinate and the elongation of the rod-like object on the abscissa, the external tensile force shows a linear locus at a fixed angle of inclination to the abscissa in the elastic area but in the plastic area it shows a locus almost parallel to the abscissa at a very small angle thereto. The same phenomenon is observed in screw tightening as well. Letting the angle of rotation of the screw be represented on the abscissa and the tightening torque on the ordinate, the torque locus is very close to the locus of the external tensile force mentioned above. This phenomenon has long been known in the art and a variety of methods have been proposed for its application to screw tightening in the actual production process.
Since the yield tightening method permits tightening screws with tension maximal to their elastic stress, as referred to above, the method is advantageous over a conventional method which tightens screws within a sufficiently safe range in the elastic area, such as a so-called torque tightening method, in that screws of a smaller cross-sectional area could be used if they are tightened with the same tension as in the above method and that the number of screws used could be reduced if their cross-sectional area is the same as in the above method. Since almost all industrial products have blocks or parts assembled together through screw tightening, it would bring about a considerable advantage in practice if the screw size or the number of screws used could be decreased by use of the yield tightening method.
However, many difficulties are encountered in actual applications of this yield tightening method. Theoretically, a point of refraction on the torque locus is surely a yield point, but it is very difficult to correctly find it out on an actual torque curve.